Opportunistic Beamforming based Hybrid Analog-Digital Multi-User mmWave Communications: Design and Analysis

In this paper, we address the problems of large training/feedback overhead and computationally intensive algorithms to determine the phase angles of the analog precoder for downlink data transmission in hybrid analog-digital (A-D) multiuser (MU) millimeter wave (mmWave) systems. We investigate the use of opportunistic beamforming (OBF) using a random analog precoder as a solution to these issues. We analyze two schemes of OBF, called fully random precoder (full RP) and channel structure-aware random precoder (CSA-RP). In the full RP scheme, we use random phase angles across the antenna array. In the CSA-RP scheme, we consider the use of structured random beams, where the phase angles of the analog precoder are chosen so as to have the same structure as the array response of the geometric channel model of mmWave systems. For the CSARP scheme, we derive the asymptotic scaling law of the average throughput as a function of the number of users, number of antennas and the SNR, as the number of users gets large. Our simulation results show that the CSA-RP scheme achieves near-optimal throughput, i.e., close to that achieved using coherent beamforming with best user selection (called the baseline (BL) scheme), via opportunistic selection among far fewer users in the system compared to the full RP scheme.